During the assembly of components on printed circuit wiring boards (PCBs), soldering fluxes are applied to the substrate board material to insure that solder firmly bonds the components to wiring traces imprinted on the PCB. Typically, these fluxes are either rosin-based or non-rosin-based. Non-rosin-based fluxes are water soluble but are not easily removed with water alone. Usually, these fluxes are removed with saponifiers. Aqueous removal of flux from PCBs through saponification has been used for a number of years and is well known in the art. Automated batch cleaning systems which use this method have been perfected to the point where they not only clean, but also measure and record PCB batch parameters such as wash time and temperature, chemistry mix, etc. One such system is the Model 6300 .mu.P manufactured by Electronic Controls Design (ECD) of Portland, Oreg. A related system but utilizing solvents rather than saponifiers is the Model 6307 of ECD, which is the subject of our copending U.S. application Ser. No. 07/513,665, filed Apr. 24, 1990, assigned to the present assignee and incorporated herein by reference. The Model 6307 uses semi-aqueous solvents and an inert gas air purge and pumping system during the solvent spray-washing cycle.
As indicated, PCBs are customarily cleaned after soldering operations to remove various contaminants caused by the soldering process. Such ionic contamination (both positive and negative ions) can cause corrosion if not removed from the PCB. Most cleaning processes used by manufacturers of PCBs result in substantially clean boards. However, in high precision electronic applications a high level of cleanliness, and a method to verify that level of cleanliness, is imperative. The standard method for testing PCB cleanliness is MIL-P-28809 (MIL-SPEC test). This test method involves measuring the resistivity of a known alcohol (typically isopropyl alcohol or "IPA") and water mixture, at a known temperature for a given PCB area, before and after the IPA mixture is exposed to a "cleaned" PCB. If the final rinse resistivity of the IPA mixture drops below 2 M.OMEGA./cm.sup.3 the PCB is considered contaminated and must be rejected. Information concerning this method can be found in any number of references, including: Batch Aqueous Cleaning--The Road to Ultra Clean PCB Assemblies, Electronic Controls Design, Inc., 1987. An accepted and typical method and apparatus for carrying out a mil-spec test on electronic assemblies is disclosed in U.S. Pat. No. 4,023,931 to Wolfgram. It involves immersing the electronic assembly in a static pool of a mixture of deionized water and isopropyl alcohol.
Previously, a direct correlation between final rinse resistivity of water and MIL-P-28809 method cleanliness test results has been difficult to achieve due to differences in ion behavior in water as distinguished from water-alcohol mixtures. To overcome this shortcoming, the system of the present invention incorporates in a batch-type cleaning system and method a water-alcohol cleanliness test cycle following a cleaning cycle and preferably also following a water rinse cycle. The invention is directed especially to a method and apparatus for testing the reliability of an aqueous cleaning process by determining the ionic contamination of PCBs following the cleaning process. The testing method is designed to utilize the same hardware and system as that used in the preceding aqueous cleaning process cycle.
The apparatus of the present invention includes an inner chamber of corrosion resistant material and an outer enclosure encasing the inner chamber. A rack within the chamber is adapted to hold a plurality of PCBs having electronic components (e.g., resistors, capacitors, inductors, ICs and the like) assembled thereon. An operator programmable microprocessor controller causes a saponifier solution to be introduced into the interior chamber. The saponifier solution is pumped through a diffusion assembly in the chamber to spray the solution onto the PCBs. Upon completion of this cleaning cycle, the saponifier solution is drained by gravity out of the chamber. This is followed by a water rinsing cycle utilizing the same hardware and spraying procedure.
Once the rosin saponification cleaning cycle and water rinsing cycle are complete, the system microprocessor controller empties the wash chamber, purges the chamber of air using an inert gas such as nitrogen, and pumps in a standard resistivity test mixture of 75% isopropyl alcohol (IPA) and 25% water. (Experimentation has shown this IPA percentage by volume to be ideal in performing the MIL-P-28809 cleanliness test outlined above and is well known in the art). The PCBs are rinsed in the test IPA mixture by the diffusion assembly at a set temperature for a minimum set time. The test IPA mixture is continuously filtered, resistivity tested and recirculated for an additional set time or until the preset resistivity level is reached, whichever occurs first. If, after the rinse-test cycle is complete, the test IPA mixture has a resistivity of .gtoreq.2 M.OMEGA./cm.sup.3, the PCBs within the chamber are determined to be clean. If the test IPA mixture has a resistivity of .ltoreq.2 M.OMEGA./cm.sup.3, the PCBs are determined to be contaminated and are rejected for re-cleaning. If the PCBs pass the test, they are then air dried and the system is reset for the next batch.
It is an object of the present invention to provide a batch cleaning system and method for PCBs which can provide as part of the system and method total PCB batch mil-spec testing of the contamination level of an alcohol/water mixture used for rinsing PCBs.
A related object is to provide a method and system as aforesaid which uses the batch cleaning system's cleaning hardware and processes to carry out the mil-spec testing as a final cycle in a multi-cycle cleaning process that may also include a prewash cycle, a wash cycle, and a water rinse cycle.
Another object is to provide a method and system as foresaid which enhances the cleanliness of the PCBs as a byproduct of the mil-spec testing.
Other objects of the invention are to provide a batch cleaner for PCBs which can monitor and record the various parameters of the cleaning, rinsing and resistivity testing cycles, and wherein the parameters of the various cycles are programmable by the operator. Other objects, features and advantages of the present invention will be apparent from the following description of a preferred embodiment thereof and from the accompanying drawings. While the present invention is described in conjunction with an aqueous cleaning system, it can also be adapted for use with a semi-aqueous or solvent cleaning system such as described in our aforementioned copending application.